The present invention relates to a method and apparatus for identifying and characterizing optimum solvents for macromolecule and nanoparticle solutes, including, for example, solutes of carbon nanotubes or graphene as well as methods of using these solvents for the manufacture of articles and materials.
There is considerable interest in finding effective solvents for certain types of macromolecules. Such solvents could be used to promote the uniform dispersion of the macromolecules, for example, separating carbon nanotubes that tend to clump in “bundles”, “ropes”, or aggregations. A more uniform dispersion of macromolecules can improve composite materials or surface coatings that use those macromolecules. An effective solvent could also be used to exfoliate macromolecules from a mass, for example, to remove individual graphene sheets from bulk graphite. An effective solvent could be used for separation of macromolecules, for example, fractional precipitation of macromolecules of different molecular weights. A true solution, enabled by an effective solvent, can provide a delivery vehicle for the macromolecules that preserves suspension of the macromolecules as well as permits various novel manufacturing techniques.
Effective solvents for many valuable macromolecules are unknown. For example, pristine single wall carbon nanotubes (SWCNT or SWNT), like most carbon allotropes, are widely believed to be insoluble in organic or aqueous solvents. Pristine means, herein, not functionalized or chemically reacted with other elements such as oxygen. Solvent-based dispersal of SWNT currently relies on adding materials to the SWNT, for example, by covalent functionalization of the SWNT or by the addition of surfactants and/or dispersants to the surface of the SWNT. Some liquids are often loosely characterized as “solvents” without specifying the state of the solute. Some solutions are colloids or dispersions. In this respect, the literature sometimes discusses carbon nanotubes suspended in a solvent, however, it is understood to those of skill in this art that these are not thermodynamically stable solute/solvent systems in which significant concentrations of the macromolecules would be suspended indefinitely.
There are a number of techniques currently used to identify solvents for a given solute including solubility parameters and surface energies. The Hansen Solubility Parameters predict the effectiveness of a solvent by examining bond energies being an intrinsic property of the solvent and solute. When corresponding bond energies of the solvent and solute are close to each other, effective solvent action is predicted. The Hildebrand Solubility Parameter is a function of “cohesive energy density”, a property intrinsic to a material and that measures an amount of energy needed to fully separate the molecules of the material. Again, solvents with a Hildebrand parameter close to the Hildebrand parameter of the target solute are expected to be effective solvents for the solute.
Both of these techniques for predicting the effectiveness of a solvent have an advantage of relying solely on intrinsic properties of the materials of the solvent and solute. For this reason, they can be implemented with a simple search of published literature for the intrinsic properties for the solvent and solute, each measured independently.
Nevertheless, these techniques have shortcomings, including, for example, the difficulty of accurately measuring the Hansen parameters and the failure of the techniques to account for some solubility influencing parameters such as molecular shape and size.
Normally, each of these techniques would be supplemented with an empirical measurement of an actual solution of the solvent and solute to determine the concentration of the solute at saturation. Such empirical measurements can be difficult to make with macromolecules that can enter into colloid-like suspensions that obscure the determination of solubility.